Synthetic wood materials, or more specifically, cellulosic-reinforced plastic composites are finding increased use in the manufacture of a variety of products; both decorative and utilitarian. Many manufacturers have determined that reinforced composites may be utilized in products traditionally constructed of natural wood with equivalent or improved results. Such composites have even been used to manufacture products not traditionally made of natural wood, such as, for example, parking stops.
There are various reasons for the increase in popularity of cellulosic-reinforced plastic composites. One reason is the decreasing amount of natural lumber available. Although technically, natural wood is a renewable resource, existing forests continue to be harvested at a rate which exceeds the growth rate of newly planted trees. As the supply of lumber, especially large, clear lumber decreases, the cost of manufacturing products using natural wood increases.
Additionally, there are increasing pressures from environmental groups and others who realize that trees are a natural resource valuable as more than a supply of lumber, and should be preserved and protected if possible. As a major portion of many cellulosic-reinforced plastic composites may optionally be derived from recycled or waste materials generated by other processes, the use of such a material may help to preserve existing natural resources.
Another reason for the increasing popularity of cellulosic-reinforced plastic composites is the properties exhibited by such a material. Many, if not all cellulosic-reinforced plastic composites are superior to natural wood in resistance to moisture and insects, and may also possess a reduced coefficient of thermal expansion while still exhibiting workability similar to that of natural wood. Such properties allow products manufactured from cellulosic-reinforced plastic composites to reside and/or be used in environments normally unfavorable to natural wood. For example, decks and other outdoor structures may be manufactured from cellulosic-reinforced plastic composites to resist rotting and termite damage, or boat docks may be constructed which are virtually impervious to water damage. Structures and other products manufactured from cellulosic-reinforced plastic composites generally may also require little or no maintenance as compared to a similarly constructed product of natural wood.
Therefore, a need exists for cellulosic-reinforced plastic composite material having good physical properties, which can thereby be incorporated into the manufacture of an increasing number of products traditionally constructed of natural wood. The manufacture of products using a cellulosic-reinforced plastic composite also requires that the material have good moldability and/or workability.
An exemplary embodiment of a cellulosic-reinforced plastic composite of the present invention exhibits excellent physical properties and moldability characteristics, therefore, allowing it to be used in a wide variety of applications and manufacturing techniques. An exemplary embodiment of a cellulosic composite material of the present invention is generally comprised of a plastic resin and a cellulosic filler material in a range of particular proportions, and may be produced in either a solid or a foamed form. Moreover, the ingredients of the composite may be recycled or virgin materials.
A multitude of plastic resins and cellulosic fillers may be combined to form an exemplary embodiment of the composite. Suitable thermoplastic resins may include: multi-layer films; high-density polyethylene (HDPE); low-density polyethylene (LDPE); polyvinyl chloride (PVC); chlorinated polyvinyl chloride (CPVC); semi-rigid polyvinyl chloride (S-RPVC); polypropylene (PP); ethyl-vinyl acetate; acrylonitrile butadiene styrene (ABS); polystyrene; and other similar or suitable polymers and copolymers. Optional thermoset materials may include: polyurethanes (e.g., isocyanates); phenolic resins; epoxy resins; unsaturated polyester; and other similar or suitable thermoset materials. Cellulosic materials acceptable for use in such a composite may include: sawdust; newspapers; alfalfa; wheat pulp; wood chips; wood fibers; wood particles; ground wood; wood flour; wood flakes; wood veneers; wood laminates; paper; cardboard; straw; cotton; rice hulls; coconut shells; corn cobs; peanut shells; bagasse; plant fibers; bamboo fiber; palm fiber; kenaf; jute; flax; and other similar or suitable cellulosic materials.
Many other materials may also be added to the composite to improve its properties or improve processing. These materials may include inorganic fillers, cross-linking agents, blowing agents, foaming agents, foam modifiers, lubricants, stabilizers, accelerators, inhibitors, enhancers, compatibilizers, thermosetting agents, process aids, weathering additives, rubber, colorants, mildew inhibitors, and other similar or suitable additives.
One problem with using a cellulosic-reinforced plastic composite, wherein the cellulosic filler material is a wood byproduct, such as, for example, wood flour, is that the cellulosic filler material tends to retain moisture. Moisture trapped in the cellulosic filler material may cause defects during the product molding process and any unencapsulated cellulosic filler material may absorb moisture after molding. Therefore, the cellulosic filler material may generally be dried prior to introduction into the molding process, or alternatively, thoroughly relieved of a substantial portion of its moisture early in the molding process. It is also important to ensure that the cellulosic filler material is substantially coated by the plastic resin during the molding process. In addition, at least one inorganic filler may be used in place of all or a portion of the cellulosic filler material to help alleviate the problems associated with cellulosic filler material moisture content.
There are a variety of ways in which the plastic resin, cellulosic filler material, and other optional materials may be caused to interact and thereby form the composite. For example, a proper ratio of each of the components may be fed via a separate hopper or similar device into a mold or molding machine, e.g., an extrusion system, during the molding operation. Alternatively, the plastic resin and optional materials may be pre-mixed. Separate hoppers or similar devices may then be used to introduce the pre-mixed materials and cellulosic filler material into a mold or molding machine during the molding operation. Another method mixes the cellulosic filler with the plastic resin (plus some or all of any optional additives, if desired) prior to introducing the mixture to the mold or molding machine. Still another method allows for proper ratios of each of a thermoplastic resin, the cellulosic filler material, and the other optional materials to be fed into a compounder. The compounder is then used to combine and melt the individual components into a pelletized feedstock, which may then be cooled and stored for later use in a molding machine. The composite produced by the compounder does not, however, have to be pelletized and stored. The composite melt may alternatively be transported from the compounder directly to an extruder or other molding machine for immediate use. Yet another method provides for the plastic resin, the cellulosic filler material, and some or all of the other optional materials to be combined in a mixer or blender. Although the blender or mixer may be heated, the components may remain unmelted during mixing. The unmelted, mixed material may then be stored for later use, or immediately transferred to a dryer to lower the moisture content of the cellulosic filler material. After drying, the unmelted, mixed material is preferably fed to a molding machine connected to the dryer, or may be placed in a compression mold. Other components may be added to the mixed, dried material at the molding machine. Furthermore, each of the aforementioned manufacturing methods may be varied.
The cellulosic-reinforced plastic composite of the present invention may be used to produce products by any of the traditional molding means, including, but not limited to, extrusion, compression molding, and injection molding. A profile die may be utilized during extrusion to shape the cellulosic-reinforced plastic composite material as desired. A packer die may be used to further compress the cellulosic-reinforced plastic composite and improve the bonding of the individual material components. Compression molding of the cellulosic-reinforced plastic composite material of the present invention may be achieved by placing a dry-blended or pelletized form of the composite into a compression mold and compressing the material under sufficient heat and pressure. Regarding compression molding, it has also been found that a variety of secondary sheet materials may be bonded to the surface of the composite material, either during the compression molding process, or afterwards by using certain adhesives or compatibilizers. Similarly, products may also be produced by traditional injection molding means, utilizing molds and molding equipment designed for the properties and characteristics of the cellulosic-reinforced plastic composite material.
In addition to the novel features and advantages mentioned above, other features and advantages will be readily apparent from the following descriptions of the drawings and exemplary embodiments.